Let's call an array arr a mountain if the following properties hold:
arr.length >= 3 There exists some i with 0 < i < arr.length - 1 such that: arr[0] < arr[1] < ... arr[i-1] < arr[i] arr[i] > arr[i+1] > ... > arr[arr.length - 1] Given an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].
Example 1:
Input: arr = [0,1,0] Output: 1 Example 2:
Input: arr = [0,2,1,0] Output: 1 Example 3:
Input: arr = [0,10,5,2] Output: 1 Example 4:
Input: arr = [3,4,5,1] Output: 2 Example 5:
Input: arr = [24,69,100,99,79,78,67,36,26,19] Output: 2
Constraints:
3 <= arr.length <= 104 0 <= arr[i] <= 106 arr is guaranteed to be a mountain array.
public class Solution {
public int PeakIndexInMountainArray(int[] arr) {
int start = 1;
int end = arr.Length-2;
while(start<=end){
int mid = start + (end-start)/2;
if(arr[mid-1]<arr[mid] && arr[mid]<arr[mid+1]){
start = mid + 1;
}
else if(arr[mid-1]>arr[mid] && arr[mid]>arr[mid+1]){
end = mid - 1;
}
else{
return mid;
}
}
return -1;
}
}Time Complexity: O(logn)
Space Complexity: O(1)
